** Shares of optical networking suppliers Corning GLW.N and Fabrinet FN.N fall premarket after J.P. Morgan downgrades

** GLW down 3.1% to $163, FN down 2.6% to $668

** J.P. Morgan cuts rating to "neutral" from "overweight"

** Says GLW valuation creates a challenging execution hurdle relative to the earnings expectations

** Adds that fiber pricing implications for GLW are likely overestimated by investors

** FN seeing inconsistent increases in customer production and orders, not yet clear when new customers will contribute meaningfully to revenue - JPM

** Believes FN share price already reflects higher expectations than what co is likely to deliver in the near term

** GLW up 92%, FN up 50.6%, YTD, as of last close